I noticed something funny. When Louis/The Keymaster is dropped off at the firehouse, he tells Egon and Janine how Gozer will come in one of the pre-chosen forms, etc. During this, he tells them how Gozer had come twice already. If the coming of Gozer is the end of the world, why would it (he? she?) need to come again, after it has already brought the end of the world twice already?

Maybe Gozer failed the first two times…like he/she/it did the third.

Later,
Turtle

It was never said that Gozer's coming always had to herald the end of the world.

Gozer was supposed to be an ancient Sumerian demi-god that could summoned by it's followers. Gozer also had more than one incarnation. Louis spoke of Gozer the Traveller, which had pre-chosen forms it could appear as and wreak havoc:

“Gozer the Traveller will come in one of the pre-chosen forms. During the rectification of the Vuldronaii the Traveller came as a very large and moving Torb. Then of course in the third reconciliation of the last of the Meketrex supplicants they chose a new form for him, that of a Sloar. Many Shubs and Zuuls knew what it was to be roasted in the depths of the Sloar that day I can tell you.”

And Gozer the Traveller appeared in the form of the girl wearing bubblewrap on top of Dana's Apartment. But why did the Traveller disappear before ending the world? Obviously the Traveller was only an incarnation only powerful enough to cause severe damage to the enemies of the party who summoned it.

But Shandor (the architect of Dana's building) wished to bring Gozer the Destructor to Earth, in order to destroy it. The Destructor's form had to be chosen by those about to be annhilated (which is why Gozer took the form of the Stay Puft Marshmallow Man to destroy the Ghostbusters.) So The Destructor must be an incarnation of Gozer powerful enough to destroy the world (and by ancient Sumerian standards, that wouldn't be very hard, because the “world” 100,000 years ago wasn't very big by population standards!)

And since Gozer the Destructor wasn't mentioned by Louis, it leads one to believe that the Destructor had never appeared on Earth before, which is why the world was still around at the time of GB1.

A simple matter of getting your incarnations confused, young padawan.
—Ecto1A

“Gozer the Gozerion, Gozer the Destructor, Vulcas Sildror, the Traveller has come!”
Dosen't this mean that the Traveller, the Destructor, and what ever Vulcas Sildror means are the same being?

Gozer the Gozerian was the god's title, Gozer the Destructor was one of it's forms, Volguus Zildrohar was probably it's Sumerian name/or title, and the Traveller was just a form it took when presenting itself to people.

Same god, different titles.
—Ecto1A

that's Volguus Zildrohar buddy. It's Latin. It means something bad.

It is Volguus Zildrohar, but still. Why would Gozer (or whatever you want to call it) come three times to bring the end of the world? And in the prision, Egon does say how this means to bring the end of the world. And Shandor did do rituals on the top of the building to bring about Armageddon.

Ok. Here we go again.

The Traveller cannot destroy the world. It was summoned twice before (that we know of.)

The Destructor can destroy the world, but wasn't summoned until Shandor built the Apartment(which acts as a PKE antenna) and performed rites in the temple part of the building to summon Gozer the Destructor (these were the “bizarre rituals intended to bring about the end of the world” Egon was talking about.)

The Traveller always has to come, but doesn't always have to take the form of the Destructor.

Satisfied?
—Ecto1A

Oh, dear now i've gone cross-eyed.

Who cares the guys in grey beat gozer, we won, now lets not talk any more of this j/k!